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Employee and Agent Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Pension benefits |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | $ 2,813.7 | $ 2,909.6 | $ 2,726.3
Defined Benefit Plan, Tax Status [Extensible Enumeration] | Qualified Plan [Member] | Qualified Plan [Member] |
Pension benefits | Pooled separate account investments | Fixed income security portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | $ 26.8 | $ 27.9 |
Pension benefits | Pooled separate account investments | U.S. large cap equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 236.5 | 290.9 |
Pension benefits | Pooled separate account investments | U.S. small/mid cap equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 98.4 | 49.5 |
Pension benefits | Pooled separate account investments | Balanced asset allocation portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 135.0 | 97.0 |
Pension benefits | Pooled separate account investments | International equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 135.9 | 141.8 |
Pension benefits | Pooled separate account investments | Real estate investment portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 133.6 | 204.1 |
Pension benefits | Single client separate account investments | U.S. government and agencies |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 93.7 | 124.9 |
Pension benefits | Single client separate account investments | States and political subdivisions |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 15.5 | 27.0 |
Pension benefits | Single client separate account investments | Corporate debt securities |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 1,387.2 | 1,394.2 |
Pension benefits | Single client separate account investments | Hedge funds |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 3.6 |  |
Pension benefits | Single client separate account investments | Mutual funds |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 109.9 |  |
Pension benefits | Single client separate account investments | Pooled separate account investment |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 423.0 | 497.9 |
Pension benefits | Single client separate account investments | Other |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 14.6 | 54.4 |
Pension benefits | Amount measured at net asset value |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 3.6 |  |
Pension benefits | Amount measured at net asset value | Single client separate account investments | Hedge funds |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 3.6 |  |
Pension benefits | Fair value hierarchy Level 1 |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 203.6 | 124.9 |
Pension benefits | Fair value hierarchy Level 1 | Single client separate account investments | U.S. government and agencies |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 93.7 | 124.9 |
Pension benefits | Fair value hierarchy Level 1 | Single client separate account investments | Mutual funds |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 109.9 |  |
Pension benefits | Fair value hierarchy Level 2 |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 2,606.5 | 2,784.7 |
Pension benefits | Fair value hierarchy Level 2 | Pooled separate account investments | Fixed income security portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 26.8 | 27.9 |
Pension benefits | Fair value hierarchy Level 2 | Pooled separate account investments | U.S. large cap equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 236.5 | 290.9 |
Pension benefits | Fair value hierarchy Level 2 | Pooled separate account investments | U.S. small/mid cap equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 98.4 | 49.5 |
Pension benefits | Fair value hierarchy Level 2 | Pooled separate account investments | Balanced asset allocation portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 135.0 | 97.0 |
Pension benefits | Fair value hierarchy Level 2 | Pooled separate account investments | International equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 135.9 | 141.8 |
Pension benefits | Fair value hierarchy Level 2 | Pooled separate account investments | Real estate investment portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 133.6 | 204.1 |
Pension benefits | Fair value hierarchy Level 2 | Single client separate account investments | States and political subdivisions |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 15.5 | 27.0 |
Pension benefits | Fair value hierarchy Level 2 | Single client separate account investments | Corporate debt securities |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 1,387.2 | 1,394.2 |
Pension benefits | Fair value hierarchy Level 2 | Single client separate account investments | Pooled separate account investment |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 423.0 | 497.9 |
Pension benefits | Fair value hierarchy Level 2 | Single client separate account investments | Other |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 14.6 | 54.4 |
Other postretirement benefits |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 102.0 | 99.8 | $ 94.4
Other postretirement benefits | Cash and cash equivalents |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 1.0 | 1.3 |
Other postretirement benefits | Fixed income security portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 48.6 | 47.9 |
Other postretirement benefits | U.S. equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 38.7 | 36.7 |
Other postretirement benefits | International equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 13.7 | 13.9 |
Other postretirement benefits | Fair value hierarchy Level 1 |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 102.0 | 99.8 |
Other postretirement benefits | Fair value hierarchy Level 1 | Cash and cash equivalents |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 1.0 | 1.3 |
Other postretirement benefits | Fair value hierarchy Level 1 | Fixed income security portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 48.6 | 47.9 |
Other postretirement benefits | Fair value hierarchy Level 1 | U.S. equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | 38.7 | 36.7 |
Other postretirement benefits | Fair value hierarchy Level 1 | International equity portfolios |  |  |
Defined benefit plan, fair value of plan assets |  |  |
Fair value of plan assets | $ 13.7 | $ 13.9 |

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